Contamination of the refrigerant-lubricating oil mixture which accumulates in the lower part or sumps of the encasements of hermetically sealed or semi-sealed motor-compressor units employed in air conditioning or heat pump service increases with the length of service. This contamination consists for the most part of electrically conductive particles such as metal chips or carbon particles and may include semi-conductive particles and charged ions from lubricant and refrigerant breakdown. When contamination reaches a sufficient level or degree it causes malfunctioning of the compressor, clogging of refrigerant lines and the undue loading and overheating of the driving motor to the extent that a hazardous condition may result. Motor manufacturers usually provide recycling overtemperature switches of the bimetal type installed in the motor stator to provide temporary shut-down of the motor until it cools sufficiently.
But such recycling switches merely prolong a hazardous condition because usually in most instances the operation of the unit is infrequently monitored. When degradation of the unit as reflected in contamination of the refrigerant-oil mixture reaches a level indicative of a hazardous condition or indicative that the unit is nearing the end of its useful life a continued shut-down of the unit is essential. On the other hand an undesirable level of contamination of the refrigerant-oil mixture may occur early in the service of the unit or at least substantially before the end of its useful life. In such instances and wherein the encasement of the unit is such that it is practical to replace the contaminated refrigerant-oil mixture, a more discriminating means for determining a lesser level of contamination and effecting in response thereto a continued shut-down of the unit is desirable.